Thin film high permeability magnetic core flux gate device for measuring a weak magnetic field

ABSTRACT

A device for measuring a weak magnetic field is disclosed. The device includes a substrate, a magnetic core having a high magnetic permeability and in the form of a surface film on a substrate, and at least two coils magnetically coupled to the magnetic core. A pulse generator is connected to one of the coils and is used to drive the magnetic core into intermittent saturation. The other coil is used to generate an output signal. The device can be used for measuring the weak magnetic fields associated with bank notes for identifying the values of the bank notes.

This is a division of application Ser. No. 121,536, filed Nov. 17, 1987now U.S. Pat. No. 4,864,238.

FIELD OF THE INVENTION

The invention relates to a device for measuring a weak magnetic fieldand particularly to a device which compensates for the terrestrialmagnetic field while providing a high signal-to-noise ratio and beingsuitable for being produced by planar micro-technology.

BACKGROUND OF THE INVENTION

Devices for measuring weak magnetic fields are used commercially. Such adevice is used in equipment for providing monetary change in order todistinguish between bank notes which may have the same size and color.The bank notes are inscribed with a magnetic ink to enable the values tobe detected. The bank notes in the monetary change dispenser aresubjected to a constant magnetic field which magnetizes the magnetic inkin a predetermined direction before the bank notes are conveyed past thedevice for measuring the weak magnetic field to be used for identifyingthe values of the bank notes.

The magnetic ink retains a residual magnetic field after passing throughthe constant magnetic field so that the characters in the magnetic inkact as tiny permanent magnets oriented in the same spatial direction.Such residual magnetic fields are weak and exhibit a magnetic flux onthe order of approximately 2.5 Webers per square millimeter. The devicefor measuring the weak magnetic field is used to measure the magneticfields produced by the characters which are acting as permanent magnets.

The inscriptions on the bank notes differ from each other in accordancewith the value of the bank note and these differences are detected inthe magnetic fields. The device should scan the bank notes withouttouching them and the distance between the device and the bank note isusually at least one millimeter. This minimal distance is desirablebecause bank notes can be crumpled and the descriptions on the banknotes should not be worn off or damaged through friction as the banknotes are conveyed past the device.

Devices for measuring low magnetic fields have other applications. Forexample, electrical meters and power meters can obtain a measurement ofelectrical current by making a measurement of the magnetic field whichis proportional to that electrical current.

U.S. Pat. No. 3,280,974 discloses a device for measuring weak magneticfields for determining the value of US bank notes particularly FIGS. 8and 9 of the patent. The patent discloses a curved fluxgate operating ona well-known principle of the saturation core probe. A conventional,rod-shaped fluxgate is well suited for measuring weak magnetic fieldsbecause of its high sensitivity due to a large number of windings andbecause of its high resolution. The conventional, rod-shaped fluxgate ishowever, less suited for this application because its configuration isnot suited for plotting weak magnetic field sources having low magneticfluxes. Such a fluxgate is disclosed in the prior art and includes amagnetic core having a very low magnetic reluctance material such as aplate made of Mu-metal. The present commercial technology presentsdifficulties in producing two identical fluxgates to compensate for theterrestrial magnetic field using a compensating circuit including twoprobes.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations of the prior art devicesby providing two identical probes for compensating for the localmagnetic field in one compensating circuit that is easily implementedand has a large signal-to-noise ratio without a significant reduction ofthe useful signal. The devices according to the invention are suitablefor being manufactured using planar micro-technology and can bemanufactured economically with high precision and reproducibility.

In one embodiment, the invention includes a substrate, a magnetic corehaving a magnetic permeability of at least about 10⁵ and defining amagnetic circuit, and at least first and second coils magneticallycoupled to the magnetic core. At least one of the coils is in the formof a flat coil. The magnetic core is in the form of a surface film on afirst surface layer on the substrate and has a maximum thickness of lessthan about ten microns. In addition, pulsing means is coupled to thefirst coil and provides electrical current pulses so that the magneticcore is intermittently driven to magnetic saturation. The second coilprovides a source of an output signal. At least one of the coils haselectrical conductors which are electrically insulated from the magneticcore and is at least partially on at least one additional surface layeron the substrate.

Steps can be taken for compensating for the possible variation indistance between the magnetic field sensor and a bank note and for theterrestrial magnetic field by the use of two devices, one for each sideof the bank note and coupled into a differential system. Producingidentical devices for such applications and tuning the two devices canbe accomplished without undue difficulty by the use of modern planarmicrotechnology. Furthermore, the use of planar micro-technology forproducing devices according to the invention enables relativelyeconomical cost for producing devices which are very small, highlyprecise and are highly reproducible.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in greater detail inconnection with the drawings in which:

FIG. 1 is a diagrammatic representation of a first embodiment of theinvention showing the spatial position of the device relative to a banknote;

FIG. 2 is a diagrammatic representation of a second embodiment of theinvention showing the spatial position of the device relative to a banknote;

FIG. 3 is a diagrammatic representation of a third embodiment of theinvention showing the relative position of the device relative to a banknote;

FIG. 4 is a circuit diagram of a plotting device for a magnetic fieldaccording to the invention;

FIG. 5 is a graph of output signals versus time for possible outputsignals for a device according to the invention;

FIG. 6 is a representative characteristic hysteresis curve along aninput signal versus time;

FIG. 7 is a graph of output signal versus time produced by an idealizedcharacteristic hysteresis curve for an input signal versus time;

FIG. 8 is a first embodiment of a magnetic core and coils according tothe invention;

FIG. 9 is a second embodiment of a magnetic core and coils according tothe invention;

FIG. 10a is a front elevational view of a diagrammatic representation ofa third embodiment of a magnetic core and coils according to theinvention;

FIG. 10b is a side elevational view of the magnetic core shown in FIG.10a.

FIG. 11 is a plan view of the device depicted in FIG. 8;

FIG. 12 is a plan view of the device depicted in FIG. 9;

FIG. 13 is a sectional view taken along the line 13--13 in FIG. 11 orFIG. 12 showing a first embodiment of a spatial configuration of theinvention;

FIG. 14 is a sectional view taken along the line 14--14 in FIG. 11, FIG.12, or FIG. 13 showing the first embodiment of the spatial configurationshown in FIG. 13;

FIG. 15 is a sectional view taken along the line 15--15 in FIG. 11 orFIG. 12 showing a second embodiment of a spatial configuration of theinvention;

FIG. 16 is a sectional view taken along the lines 16--16 in FIG. 11,FIG. 12, or FIG. 15 showing a spatial configuration of the invention;

FIG. 17a is a sectional view of an arrangement for third and fourthembodiments of spatial configurations according to the invention:

FIG. 17b is a sectional view of the arrangement shown in FIG. 17a alongthe lines 17b--17b to show the third embodiment according to theinvention; and

FIG. 18 is a sectional view of the arrangement shown in FIG. 17a alongthe line 18--18 to show the fourth embodiment according to theinvention.

Identical reference numbers designate similar parts in all the figuresof the drawings.

In the drawings, a location of a magnetized ink is represented by avector symbolizing the magnetic field M of the corresponding permanentmagnet to be measured.

DESCRIPTION OF THE INVENTION

Turning to FIGS. 1 to 3, a bank note 1 and a device 2 according to theinvention for measuring a weak magnetic field M are showndiagrammatically. Bank note 1 is shown in a horizontal plane and movesfrom left to right at a velocity v as indicated in FIGS. 1 to 3. It isassumed herein that there is magnetic ink on the bank note 1 and that ithas been magnetized in the direction of the movement of the bank note 1,i.e. that the vector representing the magnetic field M of the magnetizedink is parallel to the direction of the continued movement of the banknote 1.

The device 2 includes a magnetic core 3 made of a ferromagnetic materialand generally constitutes at least one magnetic circuit. The magneticcircuit can include a magnetic discontinuity 10. FIGS. 1 and 2 show themagnetic circuit including a magnetic discontinuity 10 while FIG. 3shows the magnetic circuit without the magnetic discontinuity. Thearrangements shown in FIGS. 1 to 3 are representative examples and it ispossible to provide any of the spatial arrangements shown in FIGS. 1 to3 with either of the two embodiments of devices 2 shown in FIGS. 1 to 3.

In each of the FIGS. 1 to 3, at least two coils 4 and 5 are wound aroundthe magnetic core 3. In each case, the coil 4 is supplied electricalcurrent pulses i from a pulse generator (not shown) at a levelsufficient to drive the magnetic core 3 to reach magnetic saturationintermittently. Also, in each of the FIGS. 1 to 3, the coil 5 is used toprovide the output signal of the device 2 and thereby functions as asensor coil. The output voltage of the coil 5 is designated as "u".

The magnetic core 3 includes a thin film. In FIG. 1 the film of themagnetic core 3 is positioned parallel to the bank note 1 so that thelength of the magnetic discontinuity 10 extends parallel to the banknote with its width "b" extending perpendicular to both the magneticfield M and the velocity v. The width of the reading track of the device2 shown in FIG. 1 is equal to "b".

In FIG. 2, the film of the magnetic core 3 is positioned perpendicularto the bank note 1 and parallel to both the magnetic field M and thevelocity v. The resulting reading track of the device 2 in FIG. 2 isextremely narrow so that a practical system would use several devices 2next to each other and parallel to each other.

In FIG. 3, the film of the magnetic core 3 is also perpendicular to thebank note 1 but perpendicular and transverse to the direction of boththe magnetic field M and the velocity v. The resulting width of thereading track is equal to the width "f" of the magnetic core 3. Inaddition, a third coil 12 is installed on the magnetic core 3. The coils5 and 12 are substantially equal in size, electrically connected inseries, and positioned so that the bottom of each is on one of the twovertical portions of the magnetic core 3. The output voltage u of theseries circuit of coils 5, 12 is the output voltage u of the device 2.The coil 4 is located on an upper portion of the magnetic core 3.

A circuit diagram of a device according to the invention is shown inFIG. 4 and includes the coils 4 and 5 coupled to each other through themagnetic core 3, a pulse generator 6, a resistor 7, a voltmeter 8 and apermanent magnet 9. The pulse generator 6 is coupled to the coil 4through the resistor 7 and supplies electrical current pulses i. Theelectrical current pulses i as a function of time can be in the form ofa saw-tooth current wave rectified by a half-wave rectifier to produce asawtooth wave in which the negative portion is equal to zero. Generally,a saw-tooth wave configuration of the electrical current i causesrelatively lower losses in the magnetic core 3 than a sinsoidal wavehaving the same amplitude. The voltmeter 8 can be an AC voltmeterconnected to the coil 5 for measuring the output voltage.

The permanent magnet 9 can be made of SmCo5 and is positioned near themagnetic core 3 so that its magnetic field Hv magnetizes the magneticcore 3 in a negative direction relative to the magnetic field Hw asindicated in FIG. 6. The position of the permanent magnet 9 can beselected so that the point of remanence and therefore the periodicstarting point of the hysteresis loop is shifted into the portion of thecharacteristic magnetization curve which is highly sensitive to amagnetic field.

In FIGS. 1 to 3, the movement of the bank note I below the device 2produces an effective output voltage of Ueff from the output voltage uas shown in FIG. 5. In particular, the broken line curve in FIG. 5relates to the devices 2 shown in FIGS. 1 and 2 while the solid linecurve in FIG. 5 relates to the device 2 shown in FIG. 3. Each of thecurves in FIG. 5 includes a plurality of irregular periods, each withits own region of constant value. FIG. 5 shows only two periods.

FIG. 6 shows several graphs including a characteristic hysteresis curvefor an induction flux B for the magnetic core 3 as a function of themagnitude cf magnetic field H. When the value of the magnetic field Hhas been reduced to zero, the induction flux B has a residual flux valueof Br. The saturation induction flux of the magnetic core 3 is equal toBs. The magnetic field Hv of the permanent magnet 9 is oriented in anegative direction so that in the absence of the magnetic field M to bemeasured, the operating point of the hysteresis curve is moved down fromBr to Bv because the induction value Bv corresponds to -Hv. In thepresence of the magnetic field M, there is a positive magnetic field sothat the operating point shifts upward along the hysteresis curve fromBv to Ba and Ba is the induction value B at the magnetic field -(Hv-M).

The rectified saw-tooth electrical current pulses i delivered by thepulse generator 6 to the coil 4 produces a proportional magnetic fieldHi which varies in time as shown in the lower portion of FIG. 6. Themagnetic field Hi has a period T corresponding to the electrical currentpulses i and a peak amplitude Hw selected to be sufficiently large sothat the magnetic core 3 reaches positive saturation. This flux in themagnetic core 3 moves up the hysteresis curve as the magnetic field Hiincreases in the positive direction to produce an induction flux Ba.This flux in the magnetic core 3 continues as the magnetic field Hiincreases and returns to the value for which the induction flux is Ba.

For each period T of the magnetic field Hi, the induction flux B in themagnetic core 3 goes through a hysteresis loop which starts at Ba for agiven magnetic field M. Thus, the induction flux B starting from Bacontinues up the right branch of the hysteresis loop to reach thesaturation induction flux Bs. The operating point then runs through thepositive saturation branch of the hysteresis loop up to the value ofHs=Hw-(Hv-M) of the magnetic field H and subsequently returns to leavethe saturation zone through the left branch of the hysteresis loop untilit returns to the starting induction flux Ba.

The operation of the characteristic hysteresis curve will now beexplained in greater detail through an idealized characteristic curveshown in FIG. 7. The left portion of the representation shown in FIG. 7is an idealized version of the characteristic hysteresis curve of FIG.6. In each partial depiction of FIG. 7, a characteristic curve for thecase that M=0 is shown in a broken line and a characteristic curve forthe case that M>0 is shown in a solid line.

As described in connection with FIG. 6, the solid line characteristiccurve in the upper left portion of FIG. 7 starts with an induction fluxBa which is equal in value to the induction flux B corresponding toH=-(Hv-M). The corresponding broken line characteristic curve startswith the induction flux Bv which is equal in value to the induction fluxB corresponding to H=-Hv because M=0 applies to this characteristiccurve.

The upper right hand of FIG. 7 shows the variation of the induction fluxB when the magnetic field Hi takes effect. This variation is shown inthe idealized characteristic hysteresis curve by straight lines andincludes positive, trapezoidal pulses which are separated from eachother in time by constant values. FIG. 7 shows a comparison between theoutput of B versus time for the broken line trapezoidal characteristiccurve and the solid line trapezoidal characteristic curve. As indicated,an existing positive magnetic field M enlarges the width of the tops ofthe trapezoidal pulses of B. The amplitudes of the trapezoidal pulsesare equal to Bs-Ba or equal to Bs-Bv. These amplitudes correspond to thevalues of P and L shown in FIG. 6. The saturation induction flux Bs isthe reference value and the two induction fluxes Ba and Bv are selectedto be as low as possible so that the amplitude of the trapezoidal pulsescan be as large as possible

In operation, the trapezoidal induction flux B in the magnetic core 3induces the output voltage u in the form of rectangular bipolar pulsesin the coil 5. The variation of the output voltage u versus time isshown in the graph at the lower right of FIG. 7. It is known that theoutput voltage u is proportional to dF/dt, where F designates themagnetic flux flowing through the coil 5. The rectangular pulses areonly present for as long as the flanks or slopes of the trapezoidalpulses of the induction B are present. At all other times, i.e. betweenthe rectangular pulses, the value of u is equal to zero because theamplitudes of the trapezoidal pulses of B are constant.

The changes in the width of the tops of the trapezoidal pulses of B dueto the presence of the magnetic field M produces a reduction in thepulse width of the rectangular pulses of u. A Fourier analysis of therectangular pulses of u indicates that the amplitude of the secondharmonic frequency is approximately proportional to the value of M or,expressed more generally, the information relating to M is contained inthe amplitudes of the even-numbered harmonic frequencies. Theodd-numbered harmonic frequencies, primarily the first harmonicfrequency, present interferences to the desired measurement.

The second harmonic frequency can be filtered at a fluxgate by means ofa filter but such a filter is generally expensive. The use of such afilter enables the value of M to be obtained by continuously measuringthe amplitude of the second harmonic frequency. The device shown in FIG.4 does not require the use of an expensive filter and enablesmeasurement of the magnetic field M produced by the magnetized ink. Thevoltmeter 8 in FIG. 4 measures the effective value of the rectangularpulses of u by indicating the mean value of the square value of theserectangular pulses. This is a typical operation of conventionallyavailable voltmeters. In this measurement, the odd-numbered harmonicfrequencies act as an interference signal and results in an offsetvoltage even when the measured value of M is zero.

The interference ratio between the first and second harmonic frequenciesis in the order of 1000:1 in a conventional cylindrical fluxgate. Incontrast, this interference ratio is only 30:1 in the device accordingto the invention because of the relatively very large reduction of thecross-section of the magnetic core. This improvement is enhanced byreducing the thickness of the film of the magnetic core 3 using chemicalpost-etching to obtain a thickness below the conventional minimal layerthickness of 0.025 mm to less than 10 microns. The use of such a smallthickness for the film has additional advantages including suppressingthe skin effect when operating with high frequency electrical currentpulses i. Furthermore, the thin film is easily magnetized.

In FIG. 8, a first embodiment of the magnetic core 3 is shown in a planview and includes a rectangular portion having a magnetic discontinuity10 of length d. The connecting portions of the magnetic core 3 whichinclude the magnetic discontinuity 10 are significantly wider than theother connecting portions of the magnetic cores 3 and each has a widthb. These dimensions are used because the magnetic core 3 is a thin film.The lower edge of the magnetic core 3 in FIG. 8 is parallel to the banknote 1 and to the vector representing the magnetic field M to bemeasured. The distance between the lower edge of the magnetic core 3 andthe bank note 1 is equal to g, which is at least one millimeter.

The weak magnetic flux lines of the magnetic field M penetrate from thelower right side as shown in FIG. 8, into the magnetic core 3 wherein aportion of the flux lines flow through the magnetic core 3 in the formof a useful magnetic flux Fm while the remainder flows in the form of anunusable leakage flux Fs through the magnetic discontinuity 10. Asmaller portion of the magnetic flux closes on itself in the spacebetween the magnetic core 3 and the bank note 1. The coils 4 and 5 arelocated on a connecting portion of the magnetic core 3 without anymagnetic discontinuity 10.

FIG. 9 shows a second embodiment of the magnetic core 3 and is somewhatsimilar to the one shown in FIG. 8. Here the magnetic core 3 includestwo magnetic circuits having a common connecting portion 11. The firstof the two magnetic circuits includes a magnetic discontinuity 10 havinga length d and the second magnetic circuit has no magneticdiscontinuity. The first magnetic circuit is arranged relative to thebank note 1 the same as the magnetic core 3 shown in FIG. 8. The coils 4and 5 are part of the second magnetic circuit. There is an additionalcoil 12 located on the common connecting portion 11 between the twomagnetic circuits and having the same dimensions as the coil 5. Coil 5is electrically connected in series to the coil 12 so that thearithmetic sum of the signals from the coils 5 and 12 are the outputsignal.

The useful magnetic flux Fm is divided into two substantially equalparts, each being Fm/2. One portion of the magnetic flux flows in thecommon connecting portion 11 while the other portion flows in the secondmagnetic circuit. The total output voltage produced by these twomagnetic flux portions is the output voltage u and corresponds to themagnetic flux Fm in the series connected coils 5 and 12.

In FIG. 9, the magnetic discontinuity 10 can be several orders ofmagnitude greater than the magnetic discontinuity 10 in FIG. 8 becausein FIG. 9 the magnetic circuit including the magnetic discontinuity 10need not be excited by electrical current pulses i so that the leakageflux Fs amounts to only a few percent instead of half the magnetic flux.Thus, the leakage Fs in FIG. 9 is far less critical than the leakageflux in FIG. 8.

This important advantage of FIG. 9 as well as the fact that theodd-numbered harmonic signals of the rectangular output pulses shown inFIG. 7 cancel themselves in the output voltage u far exceeds thedisadvantages of having the third coil 12. Furthermore, nodemagnetization of the bank note 1 occurs because there is no magneticdiscontinuity 10 in the exciting circuit and because the magneticdiscontinuity 10 with its high induction values is relatively far awayfrom the bank note 1.

The third embodiment of the magnetic core 3 shown in FIGS. 10a and 10bis similar to the embodiment shown in FIG. 8 with the difference thatthe magnetic core 3 as well as the magnetic circuit do not include amagnetic discontinuity. In addition, a third coil 12 is present. Thethree coils 4, 5 and 12 are arranged and connected as indicated in thedescription of FIG. 3. It can be seen from FIG. 10a that each of theupper halves of Fm of the magnetic fields lines produced by themagnetization of the bank note 1 extend about halfway through one of thetwo vertical connecting portions of the magnetic core 3. Thereafter, themagnetic field lines leave the vertical connecting portions at differentheights and then return to the bank note 1. The coils 5 and 12 arelocated near the bottom in proximity of the ban note 1 on the respectivevertical connecting portions so that they can be enclosed by as many ofthe magnetic field lines as possible, resulting in a maximum outputvoltage u.

It can be appreciated that as shown in FIG. 10b, the magnetic fieldlines produced by the magnetized bank note 1 are always partially closedthrough space so that a sheared permeability always exists for them.

Generally, the output voltage u of the device 2 can be increased byusing a relatively high frequency for the electrical current pulses i,e.g. about 100 kHz.

FIG. 11 shows a plan vieW of an embodiment according to the invention asdepicted in FIG. 8. FIG. 11 shows a top view of a substrate 13 which canbe made of silicon or glass. The magnetic core 3 shown in FIG. 11 hasthe same form as shown in FIG. 8 including the magnetic discontinuity 10shown in a solid line. In FIG. 11, the magnetic discontinuity 10 doesnot follow a straight path as in FIG. 8 but follows a meandering path.

The magnetic core 3 shown in FIG. 11 is fabricated on the substrate 13in the form of a thin surface film. The extreme reduction of thecross-section of the magnetic core 3 in the invention results in asimilar reduction in the demagnetization factor N and allows the use ofhighly permeable materials for the magnetic core 3 because the shearedpermeability is known to be equal to 1/(N+1/μ_(r)). The utilization ofhigh values of μ_(r) are only reasonable when the values of thedemagnetization factor N are extremely low. The ferromagnetic materialof the magnetic core 3 has a relative permeability μ_(r) which has anorder of magnitude of at least one hundred thousand (10⁵). Accordingly,it is desirable to make the ferromagnetic material magnetic glass.

The conductors of the coils 4 and 5 are electrically insulated from themagnetic core 3 and are at least partly on an additional surface layer.Only the lower conductors of the coils 4 and 5 are shown hatched in FIG.11 with no separating line indicated between the coils 4 and 5. Theconductors shown on the left side of FIG. 11 are associated with thecoil 4 and the conductors shown on the right side of FIG. 11 areassociated with the coil 5. The conductors of the coils 4 and 5 can belocated on an additional surface layer located on the substrate 13 andare, for the most part, nearly parallel to each other at least withintheir own surface layer. The relative positions of the different surfacelayers can be seen in FIGS. 13 to 16. The lower conductors of the coils4 and 5 shown in FIG. 11 are located in an additional surface layerwhich is located below the surface film of the magnetic core 3.

Generally, the length of an edge of the substrate 13 is in the order ofa few millimeters so that the lengths of the rectangular cylindricalcoils 4 and 5 on the substrate 17 are very short. The coils 4 and 5 canbe flat coils. It is known that flat coils exhibit poor winding factors,and the limited number of turns makes it desirable to use higherfrequencies for the electrical current pulses i, preferably in the orderof 100 kHz. Such a configuration of the coils results in the widening ofthe hysteresis loop so that the excitation coil 4 is preferably designedfor a minimal ampere-turns requirement.

Any magnetization which brings the magnetic core 3 up to the point ofsaturation is associated with the amount of excitation in a shearedmagnetic circuit such as a magnetic circuit having a magneticdiscontinuity. This excitation is kept low for several reasons. Thereare relatively few turns of the coil 4 which has been fashioned as aflat coil, and the magnetic resistance R of the magnetic discontinuity10 is likely to be as low as possible. In particular,

R=δ/μ_(o) bt

where:

R=magnetic resistance of a magnetic discontinuity

δ=length of the magnetic discontinuity along its path such as shown inFIG. 11;

b=the width of the magnetic discontinuity such as indicated in FIG. 11;

t=the thickness of the magnetic core 3;

μ_(o) =the permeability of the magnetic discontinuity

From the equation for R, it can be seen that a low value for thethickness t of the magnetic core 3 can be compensated for by having thewidth b of the discontinuity relatively large and/or by having themagnetic discontinuity 10 follow a meandering course in order to haveits effective length relatively large. The magnetic flux lines in FIG.11 (not shown) cross the meandering shaped magnetic discontinuity 10 atan angle α which is substantially perpendicular to the surface of theferromagnetic material of the magnetic core 3. It is known that tan α isequal to r/1=100,000/1 so that the angle α of the magnetic flux lines issubstantially 98°.

The meandering configuration of the magnetic discontinuity formed fromspacing of width d increases the effective width of the magneticdiscontinuity from b to 4(a+c) so that a very wide magneticdiscontinuity is created perpendicularly and transversely to themagnetic flux lines despite the limited width b of the correspondingconnecting portion of the magnetic core 3. As shown in FIG. 11, themeandering configuration of the magnetic discontinuity has four periods.The length of the portion of the magnetic discontinuity 10 which isparallel to the corresponding connecting portion of the magnetic core 3is designated as "a" and the portion of the magnetic discontinuityperpendicular and transverse to the corresponding connecting portion ofthe magnetic core 3 is designated as "c".

FIG. 12 shows a plan view of an embodiment of the invention depicted inFIG. 9 and is similar to FIG. 11 with the difference that the magneticcore 3 in FIG. 9 differs from the magnetic core 3 in FIG. 8. FIG. 12shows the two magnetic circuits of FIG. 9 and a magnetic discontinuity10 which is a straight line. The magnetic flux produced by theexcitation coil 4 flows only into the magnetic circuit lacking themagnetic discontinuity 10. The absence of the magnetic discontinuity 10substantially reduces the excitation required for saturation.

FIG. 12 shows the lower conductors of the coil 12 which can be locatedon the same surface layer as the lower conductors of the coils 4 and 5.At least the conductors of the coil 12 which lie within one and the samesurface layer are nearly parallel to each other. The coil 12 is alsofabricated as a flat coil.

In both FIGS. 11 and 12, the conductors of each of the coils 4, 5, and12 are located in one and the same surface layer substantiallyperpendicular and transverse to the corresponding connected portion ofthe magnetic core 3 located in another parallel surface layer.

The plan views of the embodiment shown in FIGS. 10a and 10b are similarto the plan view shown in FIG. 11 with the difference being that themagnetic discontinuity 10 is not present. The devices shown in planviews in FIGS. 11 and 12 can be fabricated either as shown in the FIGS.13 and 14, or the FIGS. 15 and 16.

The device shown in FIGS. 13 and 14 include a substrate 13, a SiO₂ layer14 which defines the first surface layer containing the conductors ofthe lower winding halves of the coils 4 and 5 (and, if appropriate, ofcoil 12), and an insulating layer 15 which defines the second surfacelayer containing the magnetic core 3. As shown in FIG. 14, individualwindings of the coils 4 and 5 (and, if applicable 12) are completed bybond wires 16 which extend out of each of the lower winding halves,traverse insulating layer 15, then go over the magnetic core 3, againtraverse the insulating layer 15 and return to the lower winding halves.

Tho device shown in FIGS. 15 and 16 also includes the substrate 13. Thesubstrate 13 has the SiO₂ layer 14, a first surface layer containing theconductors of the lower winding halves of the coils 4 and 5 (and, ifappropriate, the coil 12), the insulating layer 15, the second surfacelayer containing the magnetic core 3, another insulating layer 17 and athird surface layer containing the conductors of the upper windinghalves of the coils 4 and 5 (and, if appropriate, the coil 12). Each ofthe lower and the upper winding halves of the coils 4, 5 and 12 arelocated in separate, parallel surface layers. The first surface layer isseparated by the SiO₂ layer 14 from the surface of the semiconductorsubstrate 13 and is covered by the first insulating layer 15.

The second surface layer containing the magnetic core 3 is located onthe surface of the insulating layer 15 and is covered by the insulatinglayer 17 which defines tho third surface layer containing the upperwinding halves of the coils 4, 5 and 12. As shown in FIG. 16, theindividual windings of the coils 4, 5 and 12 are completed by theconnectors 18 which are made of an electrically conductive material andwhich, starting at a winding half, traverse both insulating layers 15and 17 to reach the connectors of the other winding halves.

The magnetic core 3 can be etched out of a piece of commerciallyobtainable metal glass by conventional photolithographic methods. Theconductors of the flat coils 4, 5 and 12 can be made of a metal such asaluminum and the insulating layer 15 and 17 can be made of aconventional polymer such as polymer SP or photographic lacquer. PolymerSP has the advantage that the surfaces of the insulating layers 15 and17 can be ground flat before a subsequent layer is applied. The coveredsubstrate 13 can have a thickness of approximately one millimeter andeach edge can be from about 4 to about 8 mm. long.

The device according to the invention can be produced by conventionalmonolithic planar technology as well as by conventional hybridtechnology. The latter technology is particularly suited for relativelylarge sizes.

Using planar technology and thin-film technology, the film thickness ofthe magnetic core 3 is preferably in the range of from about 0.1 micronto about 1 micron. The third and fourth embodiments of the deviceaccording to the invention shown in FIGS. 17a, 17b and 18 areparticularly suited for such thin films.

FIGS. 17a and 17b relate to the third embodiment of the inventiondepicted in FIG. 3 and show the coil 5 in the form of a spiral with itsconductors arranged in a single plane between the substrate 13 and themagnetic core 3 on the surface of the substrate 13. The magnetic core inthis case is in the form of a thin, oblong strip positionedsubstantially parallel to the surface of the substrate 13.

The coil 5 can take the form of a circular spiral (not shown) and thelength of the magnetic core 3 is at the most equal to one half of theouter diameter of this coil 5 so that the magnetic core 3 starts at theouter edge of the coil 5 and is arranged so that its longitudinal axislies radially with respect to the circular spiral configuration of thecoil 5.

FIG. 17a shows the coil 5 in the form of a rectangular spiral. In thiscase, two parallel outer sides of the coil 5 face each other and arelocated at a distance g (or g+k) from the outer surface of the bank note1, perpendicularly to the longitudinal axis of the magnetic core 3. Thelongitudinal axis of the magnetic core 3 is oriented vertically withrespect to the surface of the bank note 1 and the length of the magneticcore 3 is at the most equal to one half k/2 of the width k of the coil5, measured in the longitudinal direction of the magnetic core 3.

FIGS. 17a and 17b show that the coil 4 is wound around the substrate 13and arranged relative to the coil 5 and the magnetic core 3 so that theaxis of the coil 4 is substantially parallel to the axis of the magneticcore 3 and substantially perpendicular to the axis of the coil 5 whilebeing substantially parallel to the surface of the substrate 13.

In the third embodiment, the coil 5 is present only below, on one sideof the magnetic core 3, so that the magnetic coupling is generallylimited to one half of the magnetic flux emerging from the magnetic core3. In order to increase the magnetic coupling to almost 100%, astructure as shown in the fourth embodiment is desirable. In the fourthembodiment, poor magnetic coupling is limited to only the low magneticflux emerging laterally from the thin edges of the magnetic core 3 andthe amount of magnetic flux not coupled can be disregarded.

A fourth embodiment of the invention is described herein in connectionwith FIGS. 17a and 18. An additional output coil 5a is present and thecoils 5 and 5a are arranged symmetrically relative the plane of themagnetic core 3 Only one magnetic core 3 is used. The coils 5 and 5a areconnected in series in opposing directions. The additional coil 5a iselectrically insulated and applied above as shown in FIG. 18 on theright side of both the coil 5 and the magnetic core 3 on the top of thesubstrate 13. The insulating layers 14, 15 and 16 are located betweenthe coil 5, the coil 3 and the coil 5a.

Finally, the above-described embodiments to the invention are intendedto be illustrative only. Numerous alternative embodiments may be devisedby those skilled in the art without departing from the spirit and scopeof the following claims.

I claim:
 1. A device for measuring a weak magnetic flux comprisingasubstrate, a first insulating layer formed on said substrate, first andsecond sets of conductors formed on said first insulating layer, asecond insulating layer formed over said sets of conductors on saidfirst insulating layer, a magnetic core in the form of a surface filmformed on said second insulating layer and having a magneticpermeability of at least about 10⁵ and a maximum thickness of less thanabout 10 microns, a first set of bonding wires passing through saidsecond insulating layer over said magnetic core and back through saidsecond insulating layer to connect said conductors of said first set toform a first coil coupled to said magnetic core, pulsing means coupledto said first coil for providing electrical current pulses to said firstcoil so that said magnetic core is driven intermittently into magneticsaturation, and a second set of bonding wires passing through saidsecond insulating layer over said core and back through said secondinsulating layer to connect said conductors of said second set to form asecond coil coupled to said magnetic core, said second coil serving as asource of an output signal when a current signal is applied to saidfirst coil.
 2. A device for measuring a weak magnetic flux comprisingasubstrate, a first insulating layer formed on said substrate, first andsecond sets of conductors formed on said first insulating layer, asecond insulating layer formed over said sets of conductors on saidfirst insulating layer, a magnetic core in the form of a surface filmformed on said second insulating layer, said magnetic core having amagnetic permeability of at least about 10⁵ and a maximum thickness ofless than about 10 microns, a third insulating layer formed over saidmagnetic core, third and fourth sets of conductors formed on said thirdinsulating layer, a first group of conducting connectors extendingthrough said second and third insulating layers to connect said firstand third sets of conductors to form a first coil coupled to saidmagnetic core and a second group of conducting connectors extendingthrough said second and third insulating layers to connect said secondand fourth sets of conductors to form a second coil coupled to saidmagnetic core, and pulsing means coupled to said first coil forproviding electrical current pulses to said first coil so that saidmagnetic core is driven intermittently into saturation, said second coilbeing adapted to produce an output signal when an input signal isapplied to said first coil.
 3. The device of claim 1 wherein said devicefurther comprises a permanent magnet positioned relative to saidmagnetic core so that the magnetic field of said permanent magnetmagnetizes said magnetic core in a direction opposite to the magneticfield produced by said electrical current pulses, and wherein saidcurrent pulses are saw-tooth pulses.
 4. The device of claim 1 whereinsaid magnetic core does not have a magnetic discontinuity.
 5. The deviceof claim 1 wherein said magnetic core has a magnetic discontinuityhaving a meandering path.
 6. The device of claim 2 wherein said devicefurther comprises a permanent magnet positioned relative to saidmagnetic core so that the magnetic field of said permanent magnetmagnetizes said magnetic core in a direction opposite to the magneticfield produced by said electrical current pulses, and wherein saidcurrent pulses are saw-tooth pulses.
 7. The device of claim 2 whereinsaid magnetic core does not have a magnetic discontinuity.
 8. The deviceof claim 2 wherein said magnetic core has a magnetic discontinuityhaving a meandering path.